Gas recirculation method and apparatus for superheat control with gas fan static control



June 7, 1960 G. E. MooNAN 2,939,436

GAs RECIRCULATION METHOD AND APPARATUS FOR suPERHEAT CONTROL WITH GAS FAN STATIC CONTROL med ou. 2e, 1955 2 Sheets-Sheet 1 June 7, 1960 G. E. MooNAN l 2,939,436

GAS RECIRCULATION METHOD AND APPARATUS FOR SUPERHEAT CONTROL WITH GAS FAN STATIC CONTROL Filed 0G17. 26, 1955 2 Sheets-Sheet 2 f ,28 l I r illlllllllllllllllll n INVENTOR.

George E. Moonan' AT TORNEY United States Patent() GAS RECIRCULATION METHOD AND APPARA- TUS FOR SUPERHEAT CONTROL WITH GAS FAN STATIC CONTROL George E. Moonan, Crestwood, N.Y., assign-or to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Fixed oct. 26, 195s, ser. No. 542,927 4 claims. (ci. 1224419) This invention relates to apparatus for, and a method of, generating and superheatng' high pressure steam, wherein vtheV temperature of the superheated steam is maintained at a predetermined value by gas recirculation.

The invention is exemplified in a steam generating and superheating unit involving a furnace, the walls of which include steam generating tubes in which at least a predominant portion'of the total generated steam is' generated as a resul-t of radiant heat transmission from the 'furnace gases, the furnace being provided with high temperature gases by a coal burning means operating at temperatures 'above the fusion temperature of the incom bustible in the coal. furnace over a convection superheater, and a recirculated gas system withdraws lower temperature gases from a position inthe gas stream beyond the superheater and introduces the gases into the furnace in increasing volume percentage-wise as the steam generating rate decreases. In I,the operation of such a unit, fused or semifused par ticles of slag, from the fuel, may accumulate upon the screen tubes and different sections of the convection heating means such as the superheater, and such accumulations may increase to such an extent that the draft loss throughy the furnace isl materially increased. Under auch circumstances the total draft resistance may be built up; to such an extent that it exceeds the maximum static ability of the gas recirculating fan. When this occurs,'the fan may be ruined within a few minutes because of the reverse flow of gases through the fan. Additionally, there is apt to be a sudden'increase in draft loss under conditions wherein the steam load suddenly changes from, for example, 50 percent load to l100 percent load when, at the same time, the amount of recirculated gas flow called for by the control system of the unit, approaches zero as the steam load approaches 100 percent load. The draftY loss varies as the square of the change in load, and under these circumstances that would mean that the draft loss in increased four times.y This situation is apt to 'cause a reversal of flow of the recirculated gas lthrough the fan and likewise increase thev liability of ruining the fanrin the mannerabove indicated. lThis invention involves a method of, and means for, preventing the above indicated type of damage to the fan without increasing the size of the fan or increasing the power requirementsA for operating the fan. According to the invention, when the recirculated gas flow to the furnace decreases to such an extent that the fan outlet static pressure approaches, to a predetermineddegree, the gas pressure the furnace, gas is bled from the fan exi-t and recirculated to the fan inlet, this ilow of bled gases being controlled in such a manner that the minimum amount of gas handled by the fan wheel does not drop below that required for the maximum static pressure peak of the fan. According to the invention all of the gas would be recirculated'around'the wheel of the fan at maximum boiler load when the ow of recirculated gases into the furnace is zero, and no bledgas would be recirculated around the ,Y 939,43 Patented June 7, 19,610

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inA excess oftha-t delivered at fan static peak. The flow of bled gases recirculating' about the fan wheeljmay, within the scope of the invention, be controlled manually l or automatically.

The invention also involves the recirculation of gas about the fan wheel from the fan exit to the fan inlet in such a manner that the amount of gas so bled and recirculated about the fan wheel is greater than that required at maximum fan static peak, to attain an upward characteristic of fan static at maximum boiler loads.

The invention also involves means and a method of effecting the above indicated results, involving simultaneously, both the controlled inlet of cold air to the inlet of the recirculating gas fan, and the bleeding of gas from the outlet of the re'circulating gas fan to its inlet at low rates of ow of recirculated gas, in order to maintain the' fan static at, or near, itsp'eak.

The invention involves an increase in the flow of cold air from the forced draft fan of the unit into the inlet side ofgthe recirculated gas fan, and an increase of the Y flow of bled gases from the outlet of the recirculating The heating gases pass from the A gas fanl to its inlet. as the ow ratetof recirculated'gas into the furnace decreases beyond a predetermined value (which may take place simultaneously with increase in air flow), and also provides means for protecting the fan by insulating the fan fromgas .or air connection with the furnace when the pressure differential between the fan outlet and the furnace falls below a predetermined amount, for example ll/z" of water. In the practice of the invention pressure connections are added to the fan outlet and the furnace `inlet to measure the pressure differential'between the duct between the fan. outlet and the furnace inlet, which is restricted in such a manner that the predetermined resistance is achieved. As the pressure diterential remains well above 11/2 of water,

y vthe flow of recirculated gases -to the furnace is controlled fan Wheel at boiler loads requiring recirculated gasllow i or modulated from such variables as steam temperature and rate of vapor generation. As the pressure differential decreases and approaches 1% of wa-ter, a damper may be automatically opened (simultaneously with the increased gas recirculation around the fan rotor) to permit cold air from the forced draft fan to enter the recirculatingfan inlet. The amount of this opening is adjusted as required to allow sucient cold air to enter and lower the total temperature of the recirculating` gas, thus maintaining a ll/z pressureV differential between the recirculatng fan outlet and the furnace inlet. The automatic System preferably operates so that, if the differential pressurecontinues to fall, which would be a sign that the furnace resistance is increasing, and the cold air damper 'reaches its wide open position so that no further margin may be gained, then thesystem operates to render the fan motor inoperative. At the same time inlet and outlet dampers on the inlet and outlet sides of the fan are closed and cold air dampers upstream and downstream of the inlet' and outlet dampers are opened so that the fa-n is completelysurrounded with cold air. A second precaution involved in the automatic control system involves a temperature switch at the recirculating fan outlet.v If the temperature rises in this outlet, which would indicate a reversal of ow of hot gases toward the fan, the motor is automatically stopped and the fan is automatically insulated' as previously described. In addition to these features of theY automatic system an alarm is also provided in this circuit which warns the operator that the furnace resistance is dangerously high when the pressure differential approaches ll/z of water.

The invention will beconcisely set forth in the claims, but for va complete understanding of the invention, vits uses and advantages, recourse should be had tothe following description which refers to a preferred embodiment illustrated in the accompanying drawings;

A 3 Of the drawings: o' Fig. i1 isa diagrammatic representation ofia gas recirculation system embodying the invention and associated with a vapor generating and superheating unit; and

`Fig`f2` is avertcal sectionalview of the vapor generating and super'lieating unit with which thegas recirculationsystem ofFig. 1 is associated'. Y

In-Fig.y l thereis diagrammatically illustrated'a cyclone furnace 20 burning a slag forming fuel 'and discharging liighoternperature furnace gases into a Yprii'nary furnace chamber 22, from the lower part ofwhich the gases flow upwardly as. ndicatedfat 2`4- into a vvertically elongated secondaryfurnace chamber 2`5 the walls of which include steam 'generating tubes. seeonda'ry furiace olainberthe sucoeeding convectionsection includin'g'the stean1. ,s'uperheater, and the iifnent recirculated g'as systemffor control of'supferh'catedl steam temperature over alwide range of'r'ate of'vapor generation, vare constructed and arrangedin. the manner'shown and described in the pending ypatent application Serial No. 278,872, filedon March.k 27, 1952,Y by P. HQ Koch, now Patent' No. 2,884,902. and belonging to the same assignee. As disclosed in said application and illustrated in Figl V2, the products ofI combustion Apass upwardly through the secondary furnace chamber 2'5 and over asecondaryeupe heater Y 2 7 to a downflow convection gas pass2'9, then successively contact` a primary superheater 3:1 and an economizer 33, andV then ow into a duct 354 leading to an: airheater. 37, In communication vwith the-duct 35 is the inlet of a duct 28 ofV a heating gas recirculation System n i In` theillustrative steam:- generating and superheatng nnit offEig. elmentsof the superh'eater 27 -are' ar-Y rangedV as. pendent superheater platens disposed within a vertically elongated secondary. furnace chamber receivl ing` the. combustion products fromgthe cyclone f lraee, the latter being an: example of fuelY burning means'. Parts of the dependent superheater platens are disposed in apart of thesecondary furnace chamber ZS'receivin'g combustion products including high temperature heating gaseszwith particles of l'fused slag vsuspended therein'. In theillustrative unitithere isa recirculatedv gasY systemincludinga fanandfduct'work having its inlet downstream in a gas-dow sense from tlesuperheater, and having its' gas outlet in communication; with the upper partof the primary furnace chamber 22; `This' gas recirculation system audits` associated? controls operate to maintain the temperature. ofthe superheated'steam at a. predetermined, vaiie sv'sr awid'e 1aed" range, and'teacssm: plishsthis, theflow ofrecirculatedgas to the primary"fr-V nace` chamber, is` inc reaed"as' the steam load decreases, in a lower part of 'fthe load range; The `recirculated"gais s'ystenr'also functions to decrease the. ratioof furnace absorbed heat to thejt'tal h'eatabsorbed'in superheating the vapor, as the steanl demand'decreasesithis action also involving an increaseii'n'a Yproportion' ofthe total heat absorbed by the superheater.- s

Referring further to the'system shown inFigllasbeing a part of ysuch Ya unit as that |shown in the aboveidentied penning patent application, 'the recir'cltd gas system includes a recirculatinggas Y fan 26,1preferablyt'operated by. an appropriate elctricl'notor and receiving' partially cooled heatingV gases through a duct- 28, the inlet of which is in communication with`thelgas'-ow of th'eunit at a positionbeyond the lsuperheaterl The outlet ofthe fan 26 Vis connected bythe duct work co'rnponentSQ and 32 to Vvan outlet 34V communicatingwith'- thep'rini'ary furnace chamber22.

lAsthe` rateof st ean1" dmand` decreases toward alow load value, andas the 'ratei of firing yof"fthe',cyclone furl nace 24x-is dsr'essdnhe dampers 36; disposed tu the dct work component 30 'are-open and controlied or modulated,` 4to increase. the. recirculated ,Y 1gas fflow through .the

opening 34intotheiprimaryffurnace chenal?er22r The dampers 'otmayf beffoperatedthroughouta#wide Aload Vrange by appropriate control mechanism including a damper operator 38 appropriately connected to the dampers 36 vby linkage 40. The damper operator 38 is intended to respond to changes in pneumatic loading pressure in the control line 42 and the connected control line 44, leading from a controller 46 in which, or connected with which, lthere are suitable and known control dey'fices for varying the loading pressure in the lines 42 and 44 in response to the conjoint influences of a plurality of variables such as load or rate of vapor generation, and superheated steam temperature.

The controls above referred to may be consideredas the normal operative controls for" maintainingv n'al Steam temperature' substantially at'a predetermined value and, in practice, these controls may be such as those shown and described inthe pending application to Paul S. Dickey, Serial No. 260,357 and filed on December 7, i, or inthe pending patent applicationof Charles'- S. Srnith, Serial No. 199,4(16, now aba ndoned,- f1led on December 6, 1950, or appropriate gas ref :irculationl controlcomponents such as thoseindicated in the patent application of Durham andWeaven'Ser'ial'No. 322,646, ledon November 25,1952. A ll o f these applications are assigned to The Babcock 8;' Wilcox Company and/ or its wholly owned subsidiary-The Bailey Meter Company.

In general, the pertinent normaljoperative controls of the flow of recirculated gas operate to increasethe ow of recirculated gas' as thevapor generating load d ef creases toward a predetermined low load, with a mini'- muin flow of recirculated gas at a relatively highfloal'l value.

The type o funitwith which the pertinent controls` of recirculated gas ow are associated is one 'in which'the fuel burning means such vas the cyclonefurnace 2 0 operates under a positive, or super-atmospheric pressure created by such means a's Athe forced' draf t fan 5 0, the

outlet'offwhich includesdilct` work 1 00 having airhow control dampers 101 and duct work'SZ-leading to theair inlet o f an air heater` from which the air ows by appropriate lduct'wo'rk to the air inletofthe fnelbburning. means 2t). Thus there normally is a` relatively highfgas pressure a't the position ofthe recirculated gas outlet 34, into the primary furnacec hamber 2 2, which gas pressure mustbe overcome by the operation of'the recirculatedgas fan 26 in order `for the recirculated gas to flow from the fan into the primary furnace chamber. In practice, `it has been determined'thatthere must be apressure dii'erential resulting. from now ibetween the position H, or the outlet 3 4 ofthe re'c ireitintedvv gas` system into the primary furnace chamber, andthe position I adjacent the outlet ofthe fan 26 of a value greater than 1f/2f lofffwa'ter.andtojeffeotively the actual-pressure ditferential between thesepoints as" a means to indicate alow rate of ow the pressure diff ferentialrv controller F, or 54 has opposing` sides of its diaphragm lconnected by. the lines andi 5 8` to I,the outletzside of -the 26, and-to the'up'per part of `the primaryV furnace chamber, respectively, at pfc'rsitions 1I and H,r'e'spectively. Ihis'pertinent pressrediffe're tial may b'edecieased'by'an increase in dtaft'ulossthroug'lh the furnace oecasioned by abnormal slaggi'n'g" ofthe screen and v:various convection sections, and vbfylfa'stc'lc'lexi increaseiin the'rate of vporgenera'tion Whendu 'to bsiowfthe'predsfermied value; frierenaprislefsjgg verse .nsw 0f the' high tesnpsramr'e hearing gases 'from through the duct 'comptinel'lts:32am-dV 30,an"d thi'gh'trl 26: Stich a, reverse*flow`V off gases would the fan within a matter of uminutes to cause vgreatfec'c'rnorriic': loss noto'nly'to operators ofthe electrie-generatin tion with.v the steam generating unit is associated but also to the users 'ofthe power developed thereby; Such a contingency is-preventedbygthepresentinyentiont As disclosedf'inftheV4 c'opendingA applications of l(i.l

assunse' Stallkamp, Serial No; 542,926, tiled October 26, 1955, now Patent No. 2,875,736, and L. G. Troutman, Serial No. 542,925, iiled October 26, 1955, when the differential pressure between the points'I and H, measured by the differential pressure controller 54, approaches or drops below 11/2 of water, which means that the recirculated gas liow is approaching a dangerous condition, the asso'- ciated controls so change the pneumatic loading pressure in the line components 60-62 that the alarm M is sounded and, simultaneously as a result of the signaling of the relay J through the change in loading pressure in said line components, opens the damper A"l through the agency of the damper operator A', sufficiently to permit such How of the gases from the duct opening 64 in the outlet of the fau 26 and through the communicating ducts 65 to and through the duct 66 and the opening 68 in the duct 28 or in the inlet box of the fan 26 that the pressure differential between H and I is restored t0 a value well above 11/2" of Water. Simultaneously with the opening of the damper A, the damper D may be opened by the action of the damper operator D in response to the same loading influence in the line components S0- 62, 4as continued through the line component 70, the damper D being likewise closed after the pressure diierential between the points H and I has been restored to a value well above 11/2 If the differential pressure between the positions H and I continues to fall and drops below 11/2 of water while the damper A" is wide open then the pressure switch K picks up .the influence or impulse through the lines 60 and 74 from the relay J; and through the lines 72 and 74, turns olf the fan motor switch 76; closes the dampers E through the intermediacy of the damper operator 38 and its connections; closes the dampers C and opens the dampers B through the agency of the loading line 80, the control components 82 and 84, the line 86, the damper operator 88 and its connections 89-92; to insulate the fan 26 and permit the relatively high pressure cold air from the outlet of the forced draft fan 50 to ow through the duct components 100, 102 and 32 directly into the primary furnace chamber through the opening 34.

As an additional safety factor, if other controls fail, the temperature switch L, having the element 110 responsive to gas temperatures within the opening 34 or the upper part of the primary furnace chamber, operates, upon a dangerous rise in temperature, through the agency of the loading line 112 to stop the fan motor, close dampers C and E and open the dampers B through their connections which are illustrated.

The dampers which are shown in the drawing are preferably arranged so that they may be alternatively, manually or automatically operated through the use of selector switches indicated in one of the above identified pending applications, and thus, for the insulation of the recirculating gas fan other than that above referred to the operator may manually open the damper D and the dampers B to permit cold air to ow to the primary furnace and to the inlet side of the fan, and to close the dampers A, C and E, thus providing for bodies of cold air on both inlet and outlet sides of the fan, after the fan motor is stopped.

Although the invention has been described with reference to a preferred embodiment thereof, it is to be appreciated that the invention is not necessarily limited to all of the details of that embodiment. The invention is, rather, to be taken as of a scope commensurate with the scope of the sub-joined claims.

What is claimed is:

l. In a vapor generating and superheatingv unit, wall means including vapor generating tubes normally receiving heat predominantly by radiant heat transfer from high temperature furnace gases within a furnace chamber formed by the wall means, fuel burning means in the form of a cyclone furnace supplying the furnace chamber with high temperature heating gases, a forced draft fan and associatedductwork normally supplying the' tubes internally receiving the steam generated in said generating tubes and subject externally -to the iiow of said heating gases after loss of heat therefrom vin the vapor generating zone, a recirculated gas system including a fan and fan inlet ductwork communicating with the heating gas How in the gas pass downstream of the superheater and having fan outlet ductwork communicating with the furnace chamber, dampered by-pass or shunt ductwork having its inlet connected with the gas space on the outlet side of the recirculating gas fan and having its outlet connecting with the gas space on the inlet side of the fan, and a damper in said shunt ductwork to permit recirculation of the gases around the rotor of the recirculated gas fan to maintain a predetermined gas pressure differential between the furnace and the gas fan outlet.

2. In a high pressure steam generating and steam superheating unit, means defining a furnace, steam generating means including steam generating tubes along the walls of the furnace, fuel burning means for firing the furnace including a combustion air supply system having an air supply fan, a steam superheater for superheating the steam generated in said tubes, the superheater being subject to the heat from the heating gases generated in the furnace, a gas Irecirculation system including a gas recirculating fan and fan inlet ductwork communicating with the heating gas flow downstream of the superheater in a gas ow sense, the gas recirculation system also having fan outlet ductwork leading to a position in the furnace upstream of the superheater in a gas ow sense, and means including shunt ductwork having an inlet communicating with recirculated gas flow on the outlet side of the recirculated gas fan, the shunt ductwork also having an outlet communicating with the inlet side of the recirculated gas fan and providing for the recirculation back through the fan of a proportion of the gases flowing through the gas recirculation system to maintain a predetermined gas pressure differential between the furnace and the gas fan outlet.

3. In a high pressure elastic fluid generating and superheating unit, means defining a furnace, steam generating means including elastic uid generating tubes along the walls of the furnace, fuel burning means for tiring the furnace including a combustion air supply system having an air supply fan, a superheater for superheating the elastic uid generated in said tubes, the superheater being subject to the heat from the heating gases generated in the furnace, a gas recirculation system including a gas recirculating fan and fan inlet ductwork communicating with the heating gas ow downstream of the superheater in a gas flow sense, the gas recirculation system also having fan outlet ductwork leading to a position in the furnace upstream of the superheater in a gas ow sense, and means including shunt ductwork having an inlet communicating with recirculated gas ow on the outlet side of the recirculated gas fan, the shunt ductwork also having an outlet communicating with the inlet side of the recirculated gas fan and providing for the recirculation back through the fan of a Yproportion of the gases flowing through the gas reclrculation system to maintain a predetermined gas pressure differential between the furnace and the gas fan outlet.

4. In a vapor generating and superheating unit, walls including vapor generating tubes defining a furnace having a heating gas outlet in one of said walls, means for supplying heating gases to said furnace, walls forming a gas pass opening .at one end to said gas outlet, a vapor superheating tube bank in said gas pass in the path of gasflow leaving said furnace and connected for series ow of fluid from said vapor generating tubes, gas recirculation means for maintaining a predetermined vapor aiazwinzfsfy ing: ;a f gas ,fam gas, .fan inlet ductworkcommunicating with said, gas: pass downstream gas-wiseof said tubebank.

for;withdrawing,heatinggasegrv and gastan outlet duct,- work communicating with said furnace for discharging the withdrawn Agases thereto,sand meansfor increasing the ow of gases, through Athe gasfan. to increase the static piessurefthereof, ,while maintainingtherateof ow oi recirculatedgases into said furnace substantially constant, said last namedmeans including a'idampered ductopeningzatone, end tothe gasfan inlet ductworkand at its other;end to the gas :fan outletl ductwork..

` References vCited inthefle of this spatent.

UNITED ISTATES 'PATETS' 1,280,477 Hnpnw oct. 1, 19181 2,086,812 Lu1y 111'1y'13, 1937. 2,730,971- EmmerY Jn; 17,- 195ev 2,837,066" Blaskowski lune 3, 1958;

OTHER REFERENCES B'& W Bulletin .G74 of 1954, page 24. 

